Electrolytic production of percompounds



J. MULLER June 1957 x ELECTROLYTIC PRODUCTION QF PER-COMPOUNDS Filed D90. 22, 1952 I 2 Sheeis-Sheet 1 ATTORNEYS F o o o o o o 0 e o o o e o o o .o o e a .1055? nulls/F;

June 11, 1957 J. MULLER Filed Dec. 22. 1952 w 1 32 41 fi ,zZZlTfll 27 2 Sheets-Sheet 2 INVENTOR ELECTROLYTIC PRODUCTION OF PER- COMPOUNDS Josef Muller, Rheinfelden, Baden, Germany, assignor to Deutsche Goldund Silber-Scheideanstalt vormals Roessler, Frankfurt, Germany Application December 22, 1952, Serial No. 327,221

Claims priority, application Germany December 22, 1951 8 Claims. (Cl. 204-82) The present invention relates to an improved process and apparatus for carrying out .electrolyses in cells of the diaphragm type and more particularly to an improved process and apparatus especially adapted for the anodic production of persulfuric acid and salts thereof.

It is an object of the invention to provide a process and apparatus for carrying out electrolyses with high current efiiciencies.

It is a further object of the invention to provide such a process and apparatus which permit the use of extremely high current densities.

It is still a further object of the invention to provide a process and .apparatus for carrying out an electrolyses in a plurality of serially connected electrode compartments in diaphragm type ,cells in which the electrolyte is caused to flow through the series of electrode compartments by the gases evolved during the electrolyses.

It is a particular object of the invention to provide a process andapparatus for the anodic production of persulfuric acid and its salts of high purity and concentration with good current efficiencies.

In accordance with the invention, it has been discovered that electrolytic processes in which the anode and cathode are separated by a porous diaphragm can be materially improved by suitable shaping .of the diaphragm employed to enclose the anode or cathode which not only permit the use of especially 'highcurrent densities, without the necessity of providing special cooling of the electrodes involved whereby not only high yields of easily decomposed compounds such as persulfuric acid and its salts can be obtained, but also whereby high current efliciencies are obtainable. According to the invention, the diaphragms are so constructed that they notonly uniformly surround the entirely effective length of the electrode contained therein, but also that the volume of the electrode compartment bears a certain relationship to the inner surface area of the diaphragm so that whenexpressed in centimeters, the ratio of the volume of the electrode compartment to the inner surface of the diaphragm is at least 1 :4, that is so that at least 4 cm. of i phragm surface are provi d or ea h cm? of th som Partment volume It was found that the est resu s ith r spe t to curre densiti s and rapid p o u of he de ired compound ar o t ined if the rat f t electrode compartment volume to the diaphragm area lies tween 1:6 and 1:12.1 e, 6 to 1.2 cmof d aphr area are provided for every cm? of the electrode compartment volume.

While the process and apparatus according to the invention is of general applicabilityto electrolyses of aqueous liquids wherein diaphragms are employed such as, for example, anodic oxidation processes such as the production of perchloric acid, perchlorates, .perborates, perphosphoric acid and perphosphates, as Well as the oxidation of organic compounds and cathodic reduction processes such as the Production of hydrosulfite and hydrazobenzene, it will be described in the following with par- 2. ticular reference to the production of persulfuric acid for which the process and apparatus have been found particularly adapted.

The diaphragms employed according to the invention I are in the form of relatively narrow tubular elements of uniform cross-section in order to provide the required compartment volume to diaphragm area ratio. Preferably these are circular in cross-section and are mounted concentrically with respect ,to the electrode enclosed thereby, but those of oval or rectangular cross-section can also be employed provided the necessary volume to diaphragm area ratio is provided. It has been found that porous inert ceramic tubes having, for example, an inner diameter of about 6 mm. are admirably suitable as the diaphragms for the electrolytic cells according to the invention.

The use of the relatively narrow tubular diaphragms according to the invention permits the use of extremely high current densities without provisions for cooling the electrode within the diaphragm. For example, accord ing to the invention, current densities of at least 500 amps. per liter of anolyte and preferably between 1000 and 2000 amps. per liter of anolyte are employed in the anodic production of persulfuric acid. A further advantage of the relatively narrow tubular diaphragms resides in the fact that they provide a very simple way to achieve a very rapid flow of the electrolyte through the c ls i h s been found ac o d to h invention that rapid flow of the electrolyte through the space surrounded by the diaphragm produces high current efficiencies and according to the invention it was found in the production of pfirsulfuric acid that the anolyte should only be permitted to remain within the anode cell or cells at most for fifteen minutes and preferably only 4 to 8 minutes. i

The relatively narrow tubular diaphragms surrounding the anode have. been found particularly suitable for this, as the gases formed at the anode are sufiicient to cause the anolyte to flow through the anode compartment, as because of the relatively small cross-section of the tubular diaphragm, such gases carry along the anolyte to the outlet. It was found that the pumping elfect of the gases is. so energetic that further measures need not be taken to convey the anolyte even when the electrolysis is carried out Within a plurality of the tubular diaphragms conectedin series. Consequently, according to the invention, it is no longer necessary to transport the anolyte from one cell unit to the .next by a cascading arrangement whereby a considerable saving in space is obtained.

It .is furthermore essential that the electrolyte be sup-- plied to the lower end of the tubular diaphragm or diaphragms through an appropirate feeding tube especially when a plurality thereof are to be connected in series without providing gradientsin height. The electrolyte which is expelled from the upper open end of the diaphragm tubes is collected and supplied to diaphragm tubes next in the series through appropriate tubular connections so that connection of aplurality of cell units in series is easily and expediently effected.

The pumping action of the anode gases in the narrow tubes employed according to the invention can in some instances be so great that the resulting flow can only be completely utilized when a plurality of the diaphragm tubes are connected in series. On the other hand, especially when the electrolysis is to be completed in only one tube, it may be necessary to retard the flow of the electrolyte. This may be accomplished by suitable regulation of the electrolyte supplied to the lower end of the tube, for example, by regulating the flow to the inlet by an appropriate valve or by regulating overflow of the electrolyte into the inlet tube by regulating the level of the electrolyte in the supply. When less tubes can be connected in series than the number which would make full use of the pumping action of the anode gases, it is only necessary to restrict the flow of the electrolyte supplied to the first tube in such series. On the other hand, if the number of tubes connected in series is so great that the pumping action of the anode gases is insuflicient to provide the desired flow, it can be supplemented by the use of a pump.

The conveyance of the electrolyte by the pumping action of the anode gases according to the invention provides a further advantage in that it renders it possible to alter the velocity of the flow in each step of an electrolytic process carried out with a plurality of cell units connected in series simply by selecting an appropriate number of parallelly connected tubes for each cell unit. A further advantage of the process according to the invention is that it can be carried out with countercurrent flow of the anolyte and catholyte. For example, it is possible thereby to avoid the disadvantages of the previous processes for the production of persulfuric acid which are occasioned by the fact that in the latter part of the process the cathode acidis the weakest at the point at which the H504- ion requirements are the highest in the anode space.

As has been indicated above, the diaphragms employed according to the invention are rather narrow porous tubes preferably of ceramic. These narrow tubes render possible, even with rather long tubes surrounding an anode of about 1 meter long, an advantageous reduction in the thickness of the walls without danger of mechanical failure. The reduction in the thickness of the diaphragm substantially improves'the heat exchange between the anolyte and the cooled catholyte so that even with the high current densities employed according to the invention in the production of persulfuric compounds, no special cooling means need be provided in the anode space. A further result of the thinner diaphragms is that a lower voltage loss occurs in the diaphragm. It has, for example, been found in practical use that only 27% of the voltage is used for the diaphragms according to the invention.

The electrodes employed within the narrow tubular diaphragms according to the invention are advantageously in the form of wires and especially clad wires which have a core of a good conducting metal such as copper, silver or aluminum coated with, for example, platinum. It was found that anodes formed of clad wires having a good conductor as a core such as, for example, silver coated with a dense tantalum coating and wound with fine platinum wire or otherwisecoated with platinum proved especially advantageous. For example, a silver wire coated with tantalum 100 microns thick having a total diameter of 1.3 mm. and 1 meter long having its lower portion 90 cm. wound with meters of platinum wire 0.15 mm. in

diameter provides an excellent anode for the production of persulfuric acid or other compounds through electrolytic oxidation. Such an electrode only requires 10 g. of Ag, 6.3 g. of Ta and 3.78 g. of Pt. Of course, if a single length of such clad Wire is employed as the electrode, it is necessary that the end dipping in the anolyte be coated with tantalum. If desired, however, the clad anode can be U shaped so that the ends do not dip into the electrolyte and need not be clad.

The dimensions of the electrodes employed in the narrow tubular diaphragms according to the invention are advantageously such that the distance thereof from the inner wall of the diaphragm is between 0.5 to 3 mm., preferably about 2 mm. With such spacing the optimum conditions with respect to a low voltage loss and conveyance of the electrolyte by the anode gases is achieved.

For the production of persulfuric acid, it was found desirable that the anolyte be passed through the narrow tubular diaphragms containing the anodes with a velocity of 0.5 to 2 meters per minute and preferably between about 1.0 and 1.6 meters per minute. In practice it was found that seven cell units connected in series in which the tubular diaphragms and anodes each were about 1 meter long gave very excellent results in the production of persulfuric acid of high concentration.

The specific current load with reference to the area of the inner surface of the diaphragms employed according to the invention for the production of persulfuric acid advantageously amounts to 0.1 to 0.15, preferably 0.12 to 0.14 amps. per cm. and the porosity of the diaphragm is selected so that such current density is obtained with a cell voltage of at most 4.0 to 4.5 volts. The crosssection of the diaphragms for the anodes is advantageously selected Within the requirement of the proper volume to diaphragm area ratio according to the invention so that the current taken up for 1 meter length of anode cells is between 20 to 30 and preferably about 25 amps.

The process according to the invention renders it possible in the production of persulfuric acid, especially when high current densities are employed, rapidly to achieve a high end concentration, which is of advantage as it lowers the amount of steam required for the distillation thereof when it is employed in the production of hydrogen peroxide. Also, the increased rate of formation of the persulfuric acid and the relatively short time it remains in the anode compartments is of special significance with respect to the unfavorable influence of Caro acid.

The positive anolyte flow which is obtained according to the invention produces an extraordinarily low temperature sensitivity of the electrolyte, especially when a plurality of anode systems are connected in series and, furthermore, renders it possible to regulate the supply of the anolyte and the effluent persulfuric acid formed only at one point which substantially simplifies the operation of a plant operating according to the invention. Furthermore, it may be pointed out that in an electrolytic plant employing a plurality of anodes, the frequent degasification of the anolyte is advantageous and that the increase of the quantity of anolyte flow permits a more exact and less sensitive dosing of the electrolytes. Depending upon the concentration of the acid employed, the cell voltages of 4.5 and less which can be employed are extraordinarily low. This factor coupled with the fact that, as has been mentioned before, the process provides high current efiiciencies, give energy savings of 20 to 30% in comparison with the previously known processes for the production of persulfuric acid which operate at substantially lower current densities and in some instances, operate with special anode cooling.

According to the invention, a persulfuric acid containing 350 to 400 g./l. of free persulfuric acid can be obtained with current yields of over 70%. The current requirements, calculated upon 1 kg. of H202 lies under 12 kwh. and normally is about 11 kwh. The previously known processes in comparison require at least 18 kwh.

Apparatus suitable for carrying out the process according to the invention is shown by way of example in the accompanying drawings in which:

Fig. 1 diagrammatically shows a single cell provided with a tubular anode compartment according to the invention;

Fig. 2 is an enlarged diagrammatic view of a preferred form of anode employed according to the invention;

Fig. 3 diagrammatically shows an electrolytic apparatus wherein a plurality of anodic compartments are connected in a series of units.

Fig. 4 diagrammatically shows a top view of one of the distributing pipes for feeding the anolyte to the anode compartments of one unitof the series shown in Fig. 3.

Referring to Fig. 1, 1 designates the wire electrode which is surrounded by porous diaphragm tube 2 to provide an elongated narrow anode compartment. The cathode 3 which, for example, can be of lead, is provided with lead cooling coils 4. The electrodes are supported in a vessel 5 which serves to hold the catholyte. The anolyte is supplied to the bottom end of the porous diaphragm 2 through leveling tube 6 so that it will be. carried upwardly through the relatively narrow anode compartment by the anode gases formed during electrolysis. The flow of the anolyte to the anode compartment is regulated by regulating valve 15, whereas overflow 17 serves to maintain the proper catholyte level in vessel 5. The catholyte is supplied to the vessel through opening 16. The top of the electrolysing vessel closes with a cover 7 which carries a bell 8 of porcelain or inert plastic to collect the hydrogen produced at the cathode and permit its withdrawal through outlet 12. The upper end of diaphragm tube passes through cover 7 and is attached thereto by a fluid and gas tight seal 9, preferably of nonmetallic material such as a plastic. Compartment 10 which is carried on top of the cover serves to collect the anolyte thrown up from the open end of the diaphragm tube 2 by the action of the anode gases and serves to separate the anolyte from the gases. The anode gases are withdrawn from such compartment through outlet 13, whereas the degasified anolyte is withdrawn through outlet 11.

The anode wire 1 is preferably attached to a cooled bus bar 14 by soldering to insure good electrical contact. As the free upper portion of the anode wire merely serves as a conductor for the electrolysing current and not the electrolysis itself, but still is in contact with the anode products, it is of significance that this portion be maintained as cool as possible. It was found that the use of a bus bar cooled with a circulating cooling fluid effects cooling of a considerable portion of the free upper portion of the anode wire attached thereto so that heating thereof by the joulean effect is largely avoided and undesirable effects upon the anode products are also largely avoided.

A preferred form of the anode 2 is shown in Fig. 2 in which a clad wire composed of a conducting metal core 18, such as copper, silver or aluminum and tantalum cladding 19 is wound with a thin platinum wire 20.

As has already been indicated, the present invention is especially adapted for electrolytic processes in which a large number of cell units are connected in series. Figs. 3 and 4 show by way of example, an electrolytic apparatus for the practical production of persulfuric acid. In such apparatus, seven serially connected units of anode compartments according to the invention are provided within vessel 25 serving to hold the catholyte. Each unit is composed of forty parallelly connected porous diaphragms 22 about 1 meter long and having an internal diameter of about 6 mm. which serve as the anode compartments for anodes 21. The anode compartments of each unit are arranged between cathodes 23 which are cooled by cooling coils 24. The catholyte is supplied to vessel 25 through feed line 116 and is withdrawn therefrom through overflow 117. The anolyte is supplied to the anode compartments of the first unit in the series through feed line 26 which is connected to the bottom ends of the anode compartments of such unit through line 30 and distributory line 31. The upper ends of the porous diaphragms 22 pass through cover 27 and open into compartment 100 which serves to collect the anolyte thrown up from the open ends of the porous diaphragms by the anode gases and serves to separate the anolyte from such gases. The separated anode gases are withdrawn through outlet 113, whereas the anolyte is supplied to anode compartments of the succeeding unit through lines 26', 30 and distributing line 31. The succeeding anode compartment units are similarly constructed and the anolyte collected in compartments 100 is supplied to the next unit in the series in a similar manner except that in the last unit the anolyte containing persulfuric acid is withdrawn from the apparatus through line 111 ifgroworlging up in any desired manner, for example, to

A bell 2 8 is attached to the lower side of cover 27 to collect the hydrogen evolved from the cathodes associated with the anode compartment units. The collected hydrogen is withdrawn from the apparatus through line 112. The electrolysing current is supplied to the anodes through cooled bus bars 114.

It has been especially advantageous to combine the cathode gas and anode gas and anolyte collecting members as well as the diaphragm tubes and the tubes for supplying the anolyte to the diaphragm tubes with the cover for the electrolytic vessel as one structural element, as this renders it possible to assemble the electrolytic cells for carrying out the process according to the invention in a simple and reliable manner.

The process of the invention is not limited to. the particular form of apparatus described, as numerous variations are possible as long as provisions are made for the various critical features of the invention, namely, a ratio between the electrode compartment volume and the diaphragm area within the limits described, the use of high current densities, rapid positive flow of the electrolyte through the electrode compartment so that the electrolyte remains Within the electrode compartment or compartments for only a short period of time which renders it possible to avoid the use of special additional anode cooling. For example, it is not necessary that the electrodes and tubular electrode compartments be absolutely vertical, as they can be more or less inclined and can even be of angular shape.

Example An electrolytic apparatus according to Figs. 3 and 4 With seven cathodes and seven serially connected anode units composed of 40 parallelly connected porous dia-' phragms (porous porcelain tubes 9/6 mm. diameter, about one meter long) provided with 40 anode wires (platinum silver clad wire 1.2 mm. diameter with a 60 mm. platinum cladding about one meter long) is filled with 40% sulfuric acid. Through the cooling coils at the cathodes the cooling water passes with a velocity of 50 liters per minute with a temperature of about 10 C.

After turning on of the current the load is slowly regulated to about 7000 amps. whereby 0.9 liter per minute of sulfuric acid of 41.5% are supplied to the cathode compartment. To one liter of the acid which runs from the cathode compartment and still contains 40% H2804, 20 to 30 cc. of a potential value increasing solution of hydrochloric acid and ammonium thiocyanate (1.3% (NH4)CNS|0.35% HCl) are added and the resulting fluid supplied to the feed line of the first anode unit with a velocity of 0.7 liter per minute. The conveyance of the electrolyte through the cathode compartment is regulated by the ionic flow through the diaphragms which permits a running oli of only 0.7 liter acid per minute of the efiluent 0.9 liter of acid. This quantity with the potential value increasing addition is supplied to the anode units through a special feed line. From the seventh anode unit persulfuric acid with 340345 grs. HzSzOs per liter runs off with a velocity of 0.9 liter per minute. Thrs acid is recovered with a value of 36 B. and a current efliciency of 74 to 75%. At a temperature of about 19 C. the acid contains but 2 to 4 grs. of Caro acid.

The voltage of the electrolytic apparatus is about 4.2 volt with a current requirement of about 1.55 to 1.60 kwh. per one kg. H2S2Oa. Calculated upon 1 kg. of hydrogen peroxide only about 10 kwh. are needed or 3.5 kwh. for one kg. of 35 vol. percent of hydrogen peroxide respectively.

I claim:

1. In an electrolytic process for the production of a per-compound selected from the group consisting of persulfuric acid and its salts by the anodic oxidation of an anolyte containing S04 anions in an electrolyte cell provided with an anode compartment bounded by a rigid porous diaphragm, the steps which comprise passing a column of the anolyte through a rigid elongated porous tube which surrounds an elongated wire shaped anode on all sides, the inner surface of said tube being spaced from said wire shaped anode on all sides to leave an unobstructed space between the wire shaped anode and such inner surface of said tube, the column of the anolyte being passed through said unobstructed space and the volume of said tube when measured in cm. with reference to the inner surface area of such tube when measured in cm. bearing a relationship of 1 to at least 4, and subjecting said anolyte passing through said elongated tube to anodic oxidation while maintaining a current density of at least 500 amp. per liter of anolyte.

2. The process of claim 1 in which the volume of said tube when measured in cm. with reference to the inner surface area of said tube when measured in cm. bears a relationship of between 1 to 6 and l to 12.

3. The process of claim 1 in which a current density of between 1000 and 2000 amps. per liter of anolyte is maintained during the anolytic oxidation.

4. The process of claim 1 in which the anolyte is passed through the porous tube at such a velocity that it at most remains in such tube for a period of 15 minutes.

S. The process of claim 1 in which the anolyte is passed through'the porous tube at such a velocity that it remains in such tube for a period of 4 to 8 minutes.

6. The process of claim 1 in which the anolyte is passed through the porous tube at a velocity of 0.5 to 2 meters per minute.

7. The process of claim 1 in which the anolyte is passed through the porous tube at a velocity of 1.0 to 1.6 meters per minute.

8. The process of claim 1 in which the passage of the anolyte through the rigid porous tube is effected essentially only through the action of the gas formed at the anode contained in such tube.

References Cited in the file of this patent UNITED STATES PATENTS 981,900 Teichner Jan. 17,1911 1,059,809 Adolph Apr. 22, 1913 1,104,754 Burdett July 21, 1914 1,342,378 Gerstle June 1, 1920 1,477,099 vBaum Dec. 11, 1923 1,937,621 Baum Dec. 5, 1933 2,331,320 Hartzell Oct. 12, 1943 2,349,998 Trinius May 30, 1944 

1. IN AN ELECTROLYTIC PROCESS FOR THE PRODUCTION OF A PER-COMPOUND SELECTED FROM THE GROUP CONSISTING OF PERSULFURIC ACID AND ITS SALTS BY THE ANODIC OXIDATION OF AN ANOLYTE CONTAINING SO4 ANIONS IN AN ELECTROLYTE CELL PROVIDED WITH AN ANODE COMPARTMENT BOUNDED BY A RIGID POROUS DIAPHRAGM, THE STEPS WHICH COMPRISE PASSING A COLUMN OF THE ANOLYTE THROUGH A RIGID ELONGATED POROUS TUBE WHICH SURROUNDS AN ELONGATED WIRE SHAPED ANODE ON ALL SIDES, THE INNER SURFACE OF SAID TUBE BEING SPACED FROM SAID WIRE SHAPED ANODE ON ALL SIDES TO LEAVE AN UNOBSTRUCTED SPACE BETWEEN THE WIRE SHAPED ANODE AND SUCH INNER SURFACE OF SAID TUBE, THE COLUMN OF THE ANOLYTE BEING PASSED THROUGH SAID UNOBSTRUCTED SPACE AND THE VOLUME OF SAID TUBE WHEN MEASURED IN CM3 WITH REFERRENCE TO THE INNER SURFACE AREA OF SUCH TUBE WHEN MEASURED IN CM 2 BEARING A RELATIONSHIP OF 1 TO AT LEAST 4, AND SUBJECTING SAID ANOLYTE PASSING THROUGH SAID ELONGATED TUBE 